Matthew F Bieniosek

Comparison of end/side scintillator readout with digital-SiPM for ToF PET

Side-readout of scintillation light from crystal elements in PET is an alternative to conventional end-readout configurations, with the benefit of being able to provide fine depth-of-interaction (DOI) information and good energy resolution while achieving excellent timing resolution required for time-of-flight PET. In this study, the performance of discrete LYSO scintillation elements read out from the end/side with digital silicon photomultipliers (dSiPM) has been assessed. Compared to 3 × 3 × 20 mm3 LYSO crystals read out from their ends that gave a coincidence resolving time (CRT) of 162 ± 7 ps FWHM and saturated energy spectra, a side-readout configuration achieved an excellent CRT of 144 ± 2 ps FWHM after correcting for timing skews within the dSiPM and an energy resolution of 11.8 ± 0.2% without requiring energy saturation correction. On the other hand, with smaller 3 × 3 × 5 mm3 LYSO crystals that can also be tiled/stacked to provide DOI information, a timing resolution of 134 ± 6 ps was attained but produced highly saturated energy spectra.

Compact pulse width modulation circuitry for silicon photomultiplier readout

We have shown that a compact, multiplexed PWM circuit based on a gated integrator has successfully identified sub-millimeter width crystals coupled to a PS-SSPM. Multiplexing with our gated integrator front-end shows no flood map degradation in contrast to multiplexing two PS-SSPMs by shorting corresponding channels together, which suffers from signal attenuation due to the device capacitance and added noise from the two devices. In the future we plan to try the gated-integrator technique on larger PS-SSPMs with smaller crystals, and multiplex the readouts of many more PS-SSPMs. This work will facilitate the creation of a O.5mm resolution PET system with a compact PWM readout, and could lead to simplification of readout for existing detectors.

Achieving fast timing performance with multiplexed SiPMs.

Using time of flight (ToF) measurements for positron emission tomography (PET) is an attractive avenue for increasing the signal to noise (SNR) ratio of PET images. However, achieving excellent time resolution required for high SNR gain using silicon photomultipliers (SiPM) requires many resource heavy high bandwidth readout channels. A method of multiplexing many SiPM signals into a single electronic channel would greatly simplify ToF PET systems. However, multiplexing SiPMs degrades time resolution because of added dark counts and signal shaping. In this work the relative contribution of dark counts and signal shaping to timing degradation is simulated and a baseline correction technique to mitigate the effect of multiplexing on the time resolution of analog SiPMs is simulated and experimentally verified. A charge sharing network for multiplexing is proposed and tested. Results show a full width at half maximum (FWHM) coincidence time resolution of [Formula: see text] ps for a single 3 mm  ×  3 mm  ×  20 mm LYSO scintillation crystals coupled to an array of sixteen 3 mm  ×  3 mm SiPMs that are multiplexed to a single timing channel (in addition to 4 position channels). A [Formula: see text] array of 3 mm  ×  3 mm  ×  20 mm LFS crystals showed an average FWHM coincidence time resolution of [Formula: see text] ps using the same timing scheme. All experiments were performed at room temperature with no thermal regulation. These results show that excellent time resolution for ToF can be achieved with a highly multiplexed analog SiPM readout.

Methods for increasing the sensitivity of simultaneous multi-isotope positron emission tomography

Multi-isotope PET (MIP) can distinguish thebiodistributions of two simultaneously injected PET tracers, where one tracer has an isotope that is a pure positron emitter that generates two-photon coincidences, and the other emits a positron and a gamma ray in cascade yielding a triple coincidence.1 However triple coincidence detection suffers from very low sensitivity. This work presents a method to significantly enhance the sensitivity of triple coincidences for multi-isotope PET by adding an extra detector dedicated for the detection of the third prompt gamma in coincidence with the annihilation photons. We performed Monte Carlo simulations with I124, Sc-44, and F-18 isotopes, and measurements with Na-22 and Ge-68. F-18 and Ge-68 are pure positron emitters; on the other hand I124, Sc-44, and Na-22 emit 603 keV, 1156 keV and 1275 keV prompt gamma rays respectively. For the simulations, a phantom was acquired in a simulated Siemens Inveon system with 8 cm diameter, 5 cm thick detector slab of BGO placed at one end of the system to increase detection efficiency of the third gamma ray. The simulations indicate about a twofold increase in sensitivity with the extra detector added. For the measurements, we arranged two LYSO crystals coupled to Hamamatsu MMPC silicon photomultipliers (SiPM)s for the detection of 511 keV photons in coincidences and one large 8cm diameter, 2cm thick detector slab of LYSO coupled to a PMT dedicated for the detection of the 1275 keV gamma ray. The measured ratio of the triple-coincidence counts divided by the double-coincidence detection counts is 0.037±0.003, which compares well with the analytically calculated ratio of 0.042 estimated from intrinsic and geometry efficiency considerations. Furthermore, the 511 keV scintillation detectors were mounted on a linear stage that translated the detectors while acquiring double and triple coincidences counts simultaneously in order to generate one-dimensional profiles of the Na-22 and Ge-68 point sources. The triple-coincidence allows the distinction of Na-22 point source profile from the standard 511 keV double-coincidence profile of the Ge-68 source.

Point spread function for PET detectors based on the probability density function of the line segment

We propose a new approach to calculate the Point Spread Function (PSF) for PET detectors based on the probability density function (PDF) of the line segment connecting two detector elements. Positron Emission Tomography (PET) events comprise the detection and positioning of pairs of oppositely directed 511 keV photons. The most significant blurring effect in PET is the considerable size of the detector elements, which causes uncertainty in the detected positions of photons. Typically this physical blurring is modeled in the forward direction, following photons from the source to the detectors. This work presents an analytical framework for calculating this physical blurring, from the inverse approach, that is from the detector to the source. The kernel is derived from the parameterization of the line segment whose endpoints are random variables described by the intrinsic detector response function distribution. This kernel is calculated in a first order approximation, and when compared against a measured PSF profile yields less than 8% root mean square (RMS) differences. Also, from this kernel a PSF-FWHM function of the distance to the center of the scanner is derived. The ratio between the PSF-FWHM and the intrinsic detector resolution (FWHM0) agrees with the Monte Carlo simulations. For detectors whose intrinsic response functions are described by Gaussian profiles we calculated ratios 1/√2 and √5/8 at the center (R=0) and halfway from the center at ( R=system-radius/2) respectively in agreement with published values of 1/√2 and 0.85; similarly for uniform (rectangular) profiles we get 1/2 and 3/4 which are equal to published values.

A light sharing, charge multiplexed time-of-flight depth-of-interaction PET detector

Time-of-flight (TOF) and depth of interaction (DOI) measurements are both important capabilities to improve clinical PET imaging. The combination of these two measurements has been shown to improve image quality and uniformity. Current high performance TOF DOI detectors have a high level of complexity, requiring cooling, pulse shape information or increased numbers of photosensors. This works describes and characterizes an approach to TOF DOI using a two layer, 20mm thick, lutetium-yttrium oxyorthosilicate (LYSO) light sharing crystal array. The crystal array was coupled to a single ended SiPM readout with a novel binary encoded multiplexing scheme that has a 4:1 timing channel multiplexing ratio, and two position channels. Flood maps show show excellent crystal separation with a minimum ratio of distance between crystal peaks to standard deviation of crystal peaks of 9.2. The detector achieves 10mm DOI resolution with 180 +/-2ps FWHM coincident time resolution. The top and bottom layers had 170 +/- 2ps, and 185 +/-3ps FWHM coincident timing resolutions respectively. The simplicity of the readout scheme makes it a good candidate for scaling to a practical TOF DOI PET system. The detector presented in this work has single ended readout, no active cooling, a 4:1 timing channel multiplexing ratio, and requires no pulse shape information.

Analog filtering methods improve leading edge timing performance of multiplexed SiPMs

Multiplexing many SiPMs to a single readout channel is an attractive option to reduce the readout complexity of high perfromance time of flight (TOF) PET systems. However, the additional dark counts and shaping from each SiPM cause significant baseline fluctuations in the output waveform, degrading timing measurements using a leading edge threshold. This work proposes a simple analog filtering network to reduce the baseline fluctuations in highly multiplexed SiPM readouts. With 16 SiPMs multiplexed, the FWHM coincident timing resolution for single 3mm × 3mm × 20mm LYSO crystals was improved from 401 +/−4ps to 248 +/− 5ps. With 4 SiPMs multiplexed, using a 20mm length, 2 layer DOI array of LYSO crystals the mean time resolution was improved from 277 +/− 6ps to 217 +/−4ps using a ADCMP572 comparator for timing pickoff. All experiments were performed at room temperature with no active temperature regulation. This results show a promising technique for the construction of high performance multiplexed TOF PET readout systems with simple analog leading edge timing pickoff.

3D printing for cost-effective, customized, reusable multi-modality imaging phantoms

3D printing is a developing technology that allows for the rapid fabrication of arbitrary three dimensional shapes. The technology is becoming increasingly affordable, accurate and accessible to researchers. As a result they are an attractive option for research groups to quickly and cheaply fabricate multi-modality imaging phantoms. In this work one 70 mm long, 50 mm diameter and two reusable 11 mm long, 20 mm diameter cylindrical phantoms with wells for imaging agents were fabricated for use with a high resolution PET system under development at Stanford. The 3D printed phantoms were imaged using flat panel X-ray, CT, MRI and PET modalities. The phantoms show that 3D printing can be an effective technique for quickly, and cheaply fabricating customized reusable phantoms for multiple imaging modalities.

Effects of SiPM multiplexing on timing performance

Using time of flight (ToF) measurements for positron emission tomography (PET) is an attractive avenue for increasing the signal to noise (SNR) ratio of PET images. However, achieving fast timing resolution required for high SNR gain using silicon photomultipliers (SiPM) requires many resource heavy high speed readout channels. A method of multiplexing many SiPM signals into a single electronic channel would greatly simplify ToF PET readout systems. In this work the effect of multiplexing on the timing resolution of analog SiPMs is modeled, simulated and tested. The simulations and experiments show that baseline fluctuations from cumulative uncorrelated dark noise are the most important cause of timing degradation, but their effects can be mitigated with baseline correction. A charge sharing network for position sensitive multiplexing is proposed and tested. Results show a full width at half maximum (FWHM) coincidence timing resolution of 232 +/-2ps for single 3mm × 3mm × 20mm LYSO scintillation crystals with 16 SiPMs multiplexed to a single timing channel (in addition to 4 position channels). Measurements with a 4×4 array of 3mm × 3mm × 20mm LFS crystals show excellent crystal separation with a minimum ratio of distance between crystal peaks to standard deviation of crystal peaks of 11.9. The mean FWHM coincidence timing resolution of the 4×4 LFS array was 278 +/-7ps. All experiments were performed at room temperature with no thermal regulation. These results show that fast timing resolution for ToF can be achieved with highly multiplexed analog readout.

A pulse width modulation readout method for densely packed solid state photodetectors

The field of PET is moving toward systems comprising thousands of crystal elements coupled to thousands of semiconductor photodetector channels. Because of the large number of channels, developing methods for compact signal readout, while avoiding the need for new integrated circuit development is important. In this work a compact readout system for arrays of 0.5 mm × 0.5 mm × 1.0 mm LYSO crystal elements is presented. An 18×18 element crystal array was coupled to a 2×2 array of 5 mm × 5 mm position sensitive solid-state photomultipliers (PS-SSPM). Each PS-SSPM has 4 position channels. The 16 position channels of the array were multiplexed into a total of 6 readout channels. A novel version of pulse width modulation capable of high multiplexing ratios is used to readout the electronics instead of traditional analog-to-digital conversion. Each detector pulse was transformed into a digital pulse that was then readout by an FPGA-based TDC. The system successfully identified the 0.5 mm × 0.5 mm crystals at room temperature with a ratio of distance between crystal peaks to standard deviation of the peaks of greater than 3.2. The success of this simple compact readout strategy points to a practical readout strategy for PET systems comprising thousands of semiconductor photodetectors.